DE2643793A1 - PROCESS FOR GROWING MONOCRYSTALLINE EPITACTIC LAYERS FROM RARE EARTH IRON GARNET MATERIALS - Google Patents

Description

Method of growing monocrystalline epitaxial Layers of rare earth iron garnet materials

The invention relates to a method for growing a monocrystalline rare earth iron garnet material with bismuth as a single crystal or as a monocrystalline epitaxial layer by deposition from a melt with the composing components for the garnet material and a flux, one a single crystal grown by such a method and a substrate containing one by such a method grown, monocrystalline epitaxial layer carries.

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In recent years "there has been a growing interest in single crystals of iron-garnet materials of rare earths and especially in monocrystalline thin films from such garnet materials. In this description, elements with atomic numbers 21, 39 and 57 ... 71 considered rare earths. These monocrystalline thin films are used, for example, in magnetic bubble domain devices, in integrated optical circuits that are used to forward and process information-carrying light waves serve, and in devices for thermomagnetic recording and magneto-optical reading of information used. It is known from US Pat. No. 3,697,320 that it can be used to produce iron garnet single crystals several techniques out there. It has been found that processes for growing epitaxial films are particularly are suitable for producing monocrystalline layers from these iron-garnet materials. In these procedures it is possible, for example, to remove the layers by epitaxy from the liquid phase, by so-called "liquid phase epitaxy" (LPE) to breed. In these processes, the cultivation takes place in an immersion device, in which a monocrystalline garnet substrate is brought into contact with a melt that is suitable Solution of rare earth, iron oxides and possibly required

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contains other oxides. The substrate crystals are usually grown from an essentially stoichiometric melt, for example by the Czochralsky process. A solution frequently used in processes for epitaxial deposition from the liquid phase and in other flux growing processes is a flux containing FbO, possibly with the addition of PbFp and / or B ₂ O, to control the energy of dissolution, the rate of crystallization and the temperature range in which the Crystallization is carried out. A lead-containing flux has been shown to be particularly suitable for growing magneto-optically active garnet layers which contain bismuth to increase their magneto-optic effect, since this enables a low growth temperature which is necessary for sufficient bismuth to grow in the garnet film. The lower the growth temperature, the more bismuth grows in the film (see German Offenlegungsschrift 2,349,348).

However, the use of lead-containing fluxes has the disadvantage that the grown film is also lead which causes significant optical absorption. However, films with the lowest possible optical absorption requires, in particular for

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magneto-optical applications. Lead-free fluxes have been used to produce such films, but the previously known fluxes (eg BaO-BaF ₂ - B ₂ O ₃ ) usually have the disadvantage of a combination of high viscosity and high surface tension at the desired growth temperature. This prevents a clean separation of the film from the flux when the substrate carrying the epitaxial garnet layer is removed from the melt at the end of the growth, as a result of which irregularities occur in the film surface. A high viscosity can also cause irregularities in the melt and thus in the build-up of the grown film. If a growth temperature is chosen such that the flux viscosity is low enough to be useful, it is found that little or no bismuth is present in the film.

The invention is based on the object of providing a lead-free flux structure that can accommodate a large Amount of bismuth in the crystal or film to be grown enables and a correspondingly at the growth temperature has low surface tension.

The method according to the invention is now characterized in that the flux is a mixture of Bi ₂ O and Me ₂ O (where Me is at least one element from the group

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consisting of Li, Na, K, Rb and Cs) and a eutectic composition with a eutectic Temperature below the melting point of pure BipO. In the context of this invention exist mentioned combinations of fluxes with a low surface tension (e.g. a Surface tension below 180 Dyn / cnT) to grow flawless iron-garnet single crystals of rare earths with bismuth in part of the lattice sites of the rare earths Earth,

Although the mentioned fluxes are generally beneficial for flux growing processes, for example the methods described in US Pat. No. 3,697,320, they are particularly suitable for growth by epitaxy from the liquid phase.

The invention therefore relates in particular to a method in which a monocrystalline iron-garnet layer the rare earth bismuth from a melt of the composition described above through epitaxy from the liquid phase on a crystal surface is grown, which layer is the crystallographic Orientation of the mentioned crystal face shows by bringing the crystal face into contact with the melt and causing a layer of the layer composition to crystallize on the crystal face.

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In the context of the invention, flux compositions can be chosen so that growth at temperatures from 6OO ... 9OO ° C is possible.

The viscosity of the flux mixture has been found to be no more than 16 centipoise at the growth temperature. (The viscosity of pure BipO, the use of which as a flux is known from Dutch patent application 7 115 765, is only below 16 centipoise at temperatures above 900 ° C.). In one embodiment of the method according to the invention, the viscosity can be _{further reduced by adding the oxide RO 2} to the mixture (where R is at least one element selected from the group consisting of Si, Ge, Ti, Sn, Zr, Ce, Hf and Te).

Another advantage of the use of the flux according to the invention is that the accrued with it Iron-garnet layers of the rare earth bismuth have a much lower magnetic anisotropy than with With the help of lead fluxes, iron-garnet layers of the rare earth bismuth have grown. The usage of iron-garnet layers of rare earth bismuth which is low in magnetic anisotropy important in thermomagnetic recording facilities of information.

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As an example, the invention is illustrated below with reference to the figure and the examples shown in the drawing 1 ... 4 explained in more detail. Example A does not fall within the scope of the invention and is only for the sake of comparison recorded. The figure is a section through an apparatus for growing a monocrystalline Layer by epitaxy from the liquid phase.

Apparatus and method of growth

The apparatus contains an oven with a main ceramic tube 1, the length of which is 35 cm and the inner diameter of which is 6.5 cm. The oven has a three-zone heating element 1 and through the use of three variable output transformers (not shown) it is possible to obtain a desired vertical temperature gradient. The transformers are operated with a single Eurotherm controller (not shown) with the help of a thermosensor element 3 controlled near the middle heating zone. A 100 ml platinum crucible 4 stands on an aluminum base 5 but separated therefrom by three legs 6, 6a and 6b, so that there is an air space under the crucible is. A platinum wire 12, which is clamped to the top of the crucible 4, grounds the crucible, to avoid malfunctions in thermocouple 3. The position of the crucible in the furnace is such that it can hold something located under the middle of the central heating zone. Above

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The crucible has two platinum baffles and 7a. By varying the distance between them Baffle plates, the exhaust gas removal speed, the size of the air space under the crucible and the corresponding Adjusting the vertical temperature gradient it is possible through radial temperature gradient e caused convection currents in to bring a melt 8 to a minimum. These radial temperature gradients can be achieved through use good insulation of the furnace. The vertical temperature gradient is that of the top of the crucible is up to 10 ° C hotter than the bottom, depending on the size of the crucible. The convection in the melt 8 can be regulated via a substrate with a diameter of 20 mm.

Rotating the substrate in the horizontal plane during film growth can have certain advantages to have. The apparatus described can also be used for this horizontal technique.

An advantageous method for growing an epitaxial layer is as follows. The melt composition is first ^{homogenized at a temperature approximately 50 °} C. above the temperature at which the garnet crystal no longer grows. The homogenization process can be accelerated by using a platinum stirrer (not shown),

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which is fastened in place of a substrate holder 9 ″ and is usually rotated in the melt 8 for about an hour. The speed of rotation of the stirrer is controlled by an electronic system that allows rotation in one direction "up to 600 rpm, or accelerated rotation and reversal is controlled by a function generator.

The platinum stirrer is used as the homogenization is in progress he removed from the melt and the furnace temperature to the temperature required for film growth lowered. It can take half an hour for the oven to reach this temperature, like this that the melt 8 can be in a state of equilibrium. A substrate 10 that has been cleaned beforehand (using ultrasonic cleaners with organic detergents) is turned into a Brought sample clamp 11, which itself again connected to a lowering device (not shown) is.

The substrate 10 is lowered into the oven at a rate of 40 mm / min until the substrate is located directly above the surface of the melt 8. The substrate is left in this position for one to five minutes held so that it assumes the temperature of the melt 8. At the end of this period the substrate becomes 10 lowered into the melt 8 and rotated fairly quickly, so that there is no significant difference between

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see the immersion times of the upper and lower parts of the substrate when the substrate is arranged in the vertical plane.

When the substrate holder 9 is in the desired position, the lowering device is stopped and the rotation of the substrate holder 9 is continued if necessary.

When the deposition time is over, the coated substrate 10 is removed from the melt. The speed when removing the coated substrate 10 into the cooler parts of the furnace is 40 mm / min, until the substrate 10 is outside the oven and can be removed from the apparatus.

Example A.

A 3.17 µm thick layer of (BiY ^ (FeAl) ₁ -O ₁₂ was grown in five minutes on both sides of a substrate of Gd ^ Ga ₁ -O ₁ P by an immersion process, for which the above with reference to the figure A melt was prepared consisting of

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111.22 G Bi ₂ O ₃ 265.75 G PbO 0.86 G Y ₂ O ₃ 20.83 G Fe ₂ O ₃ 0.843 G Al ₂ O

15.13 (MoI. 56) 75.80 0.24 8.30 0.53

The temperature at which the layer -was was grown, approximately 802 ⁰ C. The substrate was rotated. The layer obtained in this way was epitaxial, monocrystalline, smooth, of uniform composition and thickness, and free of defects.

example 1

A 2.33 / µm thick layer of (BiYb) ₃ (FeGa) ₅ O ₁₂ was grown in ten minutes on both sides of a substrate of Gd ^ Ga ₅ O ₁₂ by an immersion process using the apparatus described above with reference to the figure was used. A melt was produced consisting of

349.47 g Bi ₂ O ₃ 73.43 (Mol.56)

20.51 g K ₂ CO ₃ 14.59

3.63 g Yb ₂ O ₃ 0.91

14,995 g Fe ₂ O ₃ 9.23

3.5 g Ga ₂ O ₃ 1.84

The substrate was not rotated. The temperature at which the layer was grown was approximately 702 ^° C. The layer obtained in this way was epitaxial, monocrystalline, smooth, of uniform composition and thickness, and free of defects.

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Example 2

A 3.12 / µm thick layer of (BiYb) ₃ (FeGa) ₅ O ₁₂ was grown in ten minutes on both sides of a substrate of Gd ^ Ga ₁ -O ₁ P by an immersion process, for which the above with reference to the The apparatus described in the figure was used. A melt was produced consisting of

349.47 g Bi ₂ O ₃ 73.31 (Mol. J6)

20.51 g K ₂ CO ₃ 14.57

0.1 g SiO ₂ 0.17

3.63 g Yb ₂ O ₃ 0.90

14,995 g Fe ₂ O ₃ 9.22

3.5 g Ga ₂ O ₃ 1.83

The temperature at which the layer was grown was approximately 696 ⁰ C. The substrate was not rotated. The layer obtained in this way was epitaxial, monocrystalline, smooth, of uniform composition and thickness, and free of defects.

Example 3

A 3 »12 / µm thick layer of (BiY) ₃ (FeGa) ₅ O ₁₂ was grown in 30 minutes on both sides of a substrate of Gd ^ GaO ₁₂ by an immersion process using the apparatus described above with reference to the figure . A melt was produced consisting of

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349.47 G Bi ₂ O ₃ 75.29 (MoI. %) 20.51 G K ₂ CO ₃ 14.96 2.07 G ^Y 2 ° 3 0.93 10,995 G Fe ₂ O ₃ 6.94 3.5 G CYes ₀ UJ 1.88

The substrate was not rotated. The temperature at at which the layer was grown was about 816 ° C. The layer obtained in this way was epitaxial, monocrystalline, smooth, of uniform composition and thickness and flawless.

Example 4

A 6.8 / µm thick layer of (BiGd) ₃ (FeAlGa) ₅ O ₁₂ was grown in 20 minutes on both sides of a substrate of (GdCa) ₃ (GaZr) ₁ -O ₁₂ by an immersion method as described above Hand of the figure described apparatus was used. A melt was produced consisting of

909.94 G ^Bi 2 ° 3 76.74 (Mol.96) 2.35 G SiO ₂ 1.54 23.66 G Na ₀ CO, 5.10 15.57 G Gd ₂ O ₃ 1.69 45.68 G Fe ₂ O ₃ 11.29 16.40 G Ga ₂ O ₃ 3.45 0.5 G Al ₂ O 0.19

The temperature at which the layer was grown was approximately 870 ° C. The substrate was not rotated.

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The layer obtained in this way was epitaxial, monocrystalline, smooth, of uniform composition and thickness and free from defects.

The optical absorption coefficients ot when the layers produced in Examples A, 1 and 2 were irradiated with light having a wavelength of 5600 Å, 6000 % and 6325, respectively, were measured. The values found are given in the table.

TABEL 1 CX (CnT ¹ )
example 1 Oi (CnT ¹ )
Example 2 1762
1310
1094 1465
1074
860 la) O <(cm " ¹ )
Example A. 5600
6000
6325 2700
2000
1600

It became the anisotropy field K_ that in the examples A, 3 and 4 produced films were measured. The values found, expressed in Erg.ecm, are in the following Given in the table.

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V Example A. Example 3 2643793
Example 4 > 10 ⁴ 9.6 χ 10 ³ 6 χ 10 ³

All measurements were carried out at room temperature.

For further details regarding the system ₂ ^ ₂ , reference is made to the phase diagrams published by EM Levin and RS Roth in "J. Research Natl. Bur. Standards, 68A (2) 198 (1964)".

Depending on the amount added, it is found that additions of Me ₂ O (Me = Li, Na, K, Rb or Cs) in Bi ₂ O lower the surface tension and the melting point of the mixture. (In this connection it should be noted that it is common practice to add alkali metal oxides in the form of alkali metal carbonates, which convert to alkali metal oxides upon heating.) A number of surface tension and melting point measurements have been made with both such mixtures and with BipO ^ -ROp- MepO mixtures carried out and the data obtained therefrom are given in Table 3. For the sake of comparison, values for a pure ΒίρΟ, flux have also been measured.

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TABLE 3

Flux Additional
Weight in % at 850 ^° C
Dyn / cm "'' k% Li ₂ O
4% Na ₂ O
4Ji Rb ₀ O
4% Cs ₂ O 171
157
152
168
169 Melting point
in ⁰ C B ₁₂ O ₃ j 213 VA K ₀ O
2% K ₂ O
4% K ₂ O
6% K ₂ O 175
164
151
141 830 Il
Il
Il
Il
Il 4% K ₀ O
k% K ₂ O
4J6 K ₂ O
k% K ₂ O 166
155
162
156 620-640
600-625
670-680
730-755
730-755 Il
Il
Il ItQjL / "* ^ - f * ^
«-J-TQ ^^ J ^ ^ - ^ ₁₁ / 166
155 < 725
660-675
655-670 Bi ₂ 0, / Si0 ₂
BipO ^ / GeOp 670
700
700
710 Bi _o 0_ / Ce0 _o
Il 620-640
720-740

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Note ; MO ₂ 98: 2 for flux-type Bi ₂ 0 ₃ / M0 ₂ in Table 3, the molar ratio of Bi ₂ O _3.

Surface tensions are only given for 850 ^° C., since the temperature coefficient of surface tension is small for all systems studied. It is characteristic that an increase in the surface tension by% was found for every 100 ^° C. temperature drop.

It is clear that the addition of alkali metal oxides provides two major benefits, there is a strong reduction of surface tension for both the Bi ₂ O as well as for the Bi _o 0, _o -R0 -Flußmittel. On a weight percent basis, K ₂ O gives the maximum reduction in surface tension. As described in Examples 1 .- .. 4, _{garnet films have successfully grown from both the Bi 2} O ₃ -Me ₂ O- and / the Bi ₂ 0 ₃ -R0 ₂ -Me ₂ O fluxes, which means that the addition of Me ₂ O actually reduced the flux adhesion to the film surface. Second, there is a substantial reduction in the film growth temperatures that can be freely applied. Since the garnet film build-up is dependent on the growth temperature, the expansion of the growth temperature range enables a greater choice in the film compositions to be grown. In particular, the _{addition of Na 2} O allows film growth temperatures ^ 200 ⁰ C lower than the melting point of the Bi ₂ O ₃ flux.

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Preferably from 1 to 6 % by weight of the flux mixture consists of Me ₂ O. When the flux contains significantly less than 1 % by weight of Me ₂ O, there is no substantial reduction in the melting point or in the surface tension of the flux with respect to a similar flux without Me ₂ O. Likewise, if the flux contains more than 6% by weight of Me ₂ O, the reduction in melting point or surface tension becomes disadvantageously small.

With the necessary changes, the same applies to the ROp addition. Preferably, ROp constitutes from 1 to 10 % of the weight of the flux mixture. If the flux contains significantly less than 1 % by weight of ROp, there is no substantial reduction in the viscosity of the flux relative to a similar flux without RO ₂ - if the flux contains more than 10 % by weight of RO ₂ , the desired one will be Result not achieved.

Substrates, which for example consist of a monocrystalline garnet and a monocrystalline epitaxial Bear iron garnet of the rare earth bismuth, which is an-

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has grown can "for example in a thermomagnetic Device as described by J.P-. Krumme et al in Appl. Phys. Letts. 20, 451 (1972), or in a magneto-optic device, or in a magneto-optic bubble domain device (See the article by G.S. Alami in I.E.E. Trans. Mag. MAG-7, 370 (1971)), or in a magnetic bubble domain device ναι the type named by A.H. Bobeck, R.F. Fischer and J.L. Smith in AIP Conference Proc. No. 5, 45 (1971) can be used.

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Claims (9)

Claims; / 1. j method for growing a monocrystalline rare earth iron garnet with bismuth, as a single crystal or as a monocrystalline epitaxial layer, by deposition from a melt which contains the composing components for the garnet material and a flux, characterized in that the flux a mixture of Bi ₂ O, and Me ₂ O (wherein Me is at least one element selected from the group consisting of Li, Na, K, Rb and Cs) and a eutectic composition having a eutectic temperature below the melting point of pure Bi ₂ O. 2. The method according to claim 1, characterized in that the flux additionally contains the oxide ROp, wherein R is at least one element selected from the group consisting of Si, Ge, Ti, Sn, Ze, Ce, Hf and Te. 3. The method according to claim 2, characterized in that the molar ratio of Bi ₂ O,: RO ₂ : Me ₂ O is 100: χ: £, where χ is such that the mixture has a viscosity of more than 16 centipoise, and y is such that the mixture has a surface tension below 180 dynes / cm -2 at the growth temperature. 4. The method according to claim 3> characterized in that RO _{2 forms} from 1 ... 10 % of the weight of the mixture. PHB 32 521 - 21 - 709819/0615 5. The method according to claim 3, characterized in that ΜβρΟ forms from 1 ... 6 % of the weight of the flux mixture. 6. The method according to any one of claims 1 ... 5, characterized in that a monocrystalline iron-garnet layer of the rare earth bismuth is grown from the melt on a crystal face of a monocrystalline substrate by epitaxy from the liquid phase, the on this The layer obtained in this manner has the crystallographic orientation of the crystal face mentioned. 7. The method according to claim 6, characterized in that the growth temperature is between 600 and 900 ^° C. 8. Monocrystalline substrate, which is a monocrystalline epitaxial layer of an iron garnet of the rare Earth bears bismuth grown by the method of claim 1. 9. Single crystal of a rare earth iron garnet bismuth grown by the method of claim 1. PHB 32 521 ^ 098 19/06